1. Field of the Invention
The invention relates generally to electrical switching apparatus and, more particularly, to an electrical switching apparatus, such as a circuit breaker having a housing and a pole shaft bearing assembly. The invention also relates to housings for circuit breakers. The invention further relates to pole shaft bearing assemblies for circuit breakers.
2. Background Information
Electrical switching apparatus, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, abnormal voltage and other fault conditions. Typically, circuit breakers include an operating mechanism which opens electrical contact assemblies to interrupt the flow of current through the conductors of an electrical system in response to such fault conditions.
Many low-voltage circuit breakers, for example, employ a molded housing having two parts, a first half or front part (e.g., a molded cover), and a second half or rear part (e.g., a molded base). The operating mechanism for such circuit breakers is often mounted to the front part of the housing, and typically includes an operating handle and/or button(s) which, at one end, is (are) accessible from the exterior of the molded housing and, at the other end, is (are) coupled to a pivotable pole shaft. Electrical contact assemblies, which are also disposed within the molded housing, generally comprise a conductor assembly including a movable contact assembly having a plurality of movable contacts, and a stationary contact assembly having a plurality of corresponding stationary contacts. The movable contact assembly is electrically connected to a generally rigid conductor of the conductor assembly by flexible conductors, commonly referred to as shunts. The movable contact assembly includes a plurality of movable contact arms or fingers, each carrying one of the movable contacts and being pivotably coupled to a contact arm carrier. The contact arm carrier is pivoted by a protrusion or arm on the pole shaft of the circuit breaker operating mechanism to move the movable contacts into and out of electrical contact with the corresponding stationary contacts of the stationary contact assembly. The contact arm carrier includes a contact spring assembly structured to bias the fingers of the movable contact assembly against the stationary contacts of the stationary contact assembly in order to provide and maintain contact pressure when the circuit breaker is closed, and to accommodate wear.
Each of the components of the circuit breaker, including the two parts or halves (e.g., the molded cover and the molded base) of the circuit breaker housing, is subject to dimensional variation during manufacturing. Specifically, molded components, such as the molded cover and molded base, have a parting line which is created as part of the molding process, and which results in one portion (e.g., the exterior side) varying in dimension with respect to another portion (e.g., the interior side) of the same component. Such variations are also cumulative when the parts are assembled. For example, as previously noted, the operating mechanism of known low-voltage circuit breakers is mounted to the front part of the housing, which in turn is coupled to the rear part of the housing to which the stationary contact assembly is coupled. Thus, the parts are connected or “stacked” in series. Variations among the parts within the series add up, resulting in an undesirable reduction of the accuracy of the relationship (i.e., alignment) between parts across the stack.
The two separate half structures of the circuit breaker molded housing are particularly susceptible to misalignment. Specifically, variations across the parting line (the line designating the two halves of the mold used to make the component) as well as variations across the mating line or lines between components in the stack, result in misalignment between, for example, the stationary contact assembly and the pole shaft of the operating mechanism, thus inhibiting circuit breaker performance. The accuracy with which the components of the circuit breaker are mounted with respect to one another significantly affects the kinematics of the circuit breaker, and the predictable and thus repeatable mechanical, electrical and thermal performance of the circuit breaker. Accordingly, there is a need for a cost-effective circuit breaker design structured to reduce the aggregate dimensional variation among components of the circuit breaker.
It is known that the effects of dimensional variations between circuit breaker components such as, for example, between the stationary contact assembly and the operating mechanism and pole shaft, can, in large part, be minimized by reducing manufacturing tolerances. However, this approach would significantly increase manufacturing cost.
There is, therefore, room for improvement electrical switching apparatus, such as low-voltage circuit breakers, and in housings for circuit breakers and in mounting assemblies for circuit breaker components.